Radon is a link in the chain of decay of uranium-238 and occurs naturally in soil as a radionuclide gas that dissipates by exposure of soil to the atmosphere. Background readings of radioactivity of radon are reported to average about 0.25 pCi per liter by the Environmental Protection Agency. (See, Radon Reduction Techniques for Detached Houses, Technical Guidance, EPA/625/5-86/019, pg. 2). In contrast, undiluted soil gas reported by the same publication ranges from "a few hundred to several thousand pCi per liter" (Ibid). Lung cancer has been associated with the presence of radon gas: the risk of occurance ranges up to 75 times the normal risk of lung cancer (pg. 3).
Radon gas permeates construction materials such as porous cement blocks or poured foundations which have become porous over time or which have cracked due to stress. Radon can, therefore, penetrate building structures at high concentrations and consequently collect in enclosed areas. Although radon has a half-life of approximately 3.8 days and ultimately will either be ventilated outside the structure or decay, elevated concentrations of radon will persist indefinitely by continuous decay of uranium in soil and permeability of the soil to radon gas. A continuing high risk of lung cancer thus exists to occupants exposed over long periods of time.
Common methods used to-date to control the presence of radon have included use of filters, sealing points of entry within the structure, and better ventilation to increase the rate at which the volume of air contained within the structure is replaced. Filters have not been proven to be effective for removal of decay products such as radon because as a gas, the predominant portion of the gas remains unattached to particulates which can be collected by air cleaners. Entry of radon can also be reduced by application of sealing materials at points of entry in foundation structures. Points of entry typically include wall and floor joints, settling cracks, utility penetrations such as cable connections, and the porous nature of concrete. However, application of such methods are of only limited use and are not usually sufficient to remove radon which is penetrating structures except in a few specific applications. Further, sealants deteriorate and cracks and fissures tend usually to propagate, thereby minimizing the effectiveness of seals.
Ventilation has proven to be the most reliable and universally applicable means for continuously removing radon gas from subterranean foundation structures. Air captured within buildings is replaced according to patterns of usage of the building by the occupants and by the type of building design and materials selected for construction. Ventilation of residential homes historically has been passive. However, improved insulation and installation of climate control systems has reduced the rate of air replacement, thus increasing the need for active ventilation to dissipate radon gas entering living quarters. Several methods have been devised which ventilate radon-containing gas to the atmosphere from points of entry at the foundation.
Typical of radon ventilation techniques are fan-driven methods drawing radon-containing air from soil adjacent the subterranean structure into a conduit for discharge to the atmosphere. While fan-driven displacement of air is substantially more effective than either sealing techniques or filtration of captive air, there are severe limitations in that the low pressure difference generated by ventilation fans requires large volumes of air to conduct radon from within the structure. Extensive modifications must usually be made to adapt existing foundations for collection of radon-containing gas dissipating from surrounding earthen excavations. Examples of modifications which must be made to existing foundation structures in order to draw radon from beneath such structures by use of fans include removal of existing concrete slabs and deposition of aggregate over which a liner is laid, followed by poring a restored concrete floor. Another example is removal of the existing slab followed by laying perforated pipes within an aggregate bed over which a new slab floor is pored. However, removal and replacement of existing slab concrete is expensive, difficult and may detrimentally effect the structural integrity of the foundation and of the building supported above it. Further, piping and aggregate beds eventually may become blocked or filled with surrounding earth caused by water drainage patterns around the foundation. Thus, a more durable and inexpensive system is needed for installation in existing structures.